Secondary forests are becoming increasingly widespread in the tropics, but our understanding of how secondary succession affects carbon (C) cycling and C sequestration in these ecosystems is limited. We used a well-replicated 80-year pasture to forest successional chronosequence and primary forest in Puerto Rico to explore the relationships among litterfall, litter quality, decomposition, and soil C pools. Litterfall rates recovered rapidly during early secondary succession and averaged 10.5 (± 0.1 SE) Mg/ha/y among all sites over a 2-year period. Although forest plant community composition and plant life form dominance changed during succession, litter chemistry as evaluated by sequential C fractions and by 13C-nuclear magnetic resonance spectroscopy did not change significantly with forest age, nor did leaf decomposition rates. Root decomposition was slower than leaves and was fastest in the 60-year-old sites and slowest in the 10- and 30-year-old sites. Common litter and common site experiments suggested that site conditions were more important controls than litter quality in this chronosequence. Bulk soil C content was positively correlated with hydrophobic leaf compounds, suggesting that there is greater soil C accumulation if leaf litter contains more tannins and waxy compounds relative to more labile compounds. Our results suggest that most key C fluxes associated with litter production and decomposition re-establish rapidly—within a decade or two—during tropical secondary succession. Therefore, recovery of leaf litter C cycling processes after pasture use are faster than aboveground woody biomass and species accumulation, indicating that these young secondary forests have the potential to recover litter cycling functions and provide some of the same ecosystem services of primary forests.

Tropical small mountainous rivers (SMRs) may transport up to 33% of the total
carbon (C) delivered to the oceans. However, these fluxes are poorly quantified and
historical records of land-ocean carbon delivery are rare. Corals have the potential to
provide such records in the tropics because they are long-lived, draw on dissolved
inorganic carbon (DIC) for calcification, and isotopic variations within their skeletons are
useful proxies of palaeoceanographic variability. The ability to quantify riverine C inputs
to the coastal ocean and understand how they have changed through time is critical to
understanding global carbon budgets in the context of modern climate change. A seasonal
dual isotope (13C & 14C) characterization of the three major C pools in two SMRs and
their adjacent coastal waters within Puerto Rico was conducted in order to understand the
isotope signature of DIC being delivered to the coastal oceans. Additionally a 56-year
record of paired coral skeletal C isotopes (δ13C & Δ14C) and trace elements (Ba/Ca,
Mn/Ca, Y/Ca) is presented from a coral growing ~1 km from the mouth of an SMR. Four
major findings were observed: 1) Riverine DIC was more depleted in δ13C and Δ14C than
seawater DIC, 2) the correlation of δ13C and Δ14C was the same in both coral skeleton
and the DIC of the river and coastal waters, 3) Coral δ13C and Ba/Ca were annually
coherent with river discharge, and 4) increases in coral Ba/Ca were synchronous with the
iii
timing of depletions of both δ13C and Δ14C in the coral skeleton and increases in river
discharge. This study represents a first-order comprehensive C isotope analysis of major
C pools being transported to the coastal ocean via tropical SMRs. The strong coherence
between river discharge and coral δ13C and Ba/Ca, and the concurrent timing of increases
in Ba/Ca with decreases in δ13C and Δ14C suggest that river discharge is simultaneously
recorded by multiple geochemical records. Based on these findings, the development of
coral-based proxies for the history of land-ocean carbon flux would be invaluable to
understanding the role of tropical land-ocean carbon fluxes in the context of global
climate change.

Hurricane activity is predicted to increase over the mid-Atlantic as global temperatures
rise. Nitrous oxide (N2O), a greenhouse gas with a substantial source from tropical soils,
may increase after hurricanes yet this effect has been insufficiently documented. On
September 21, 1998, Hurricane Georges crossed Puerto Rico causing extensive
defoliation. We used a before–after design to assess the effect of Georges on N2O
emissions, and factors likely influencing N2O fluxes including soil inorganic nitrogen
pools and soil water content in a humid tropical forest at El Verde, Puerto Rico.
Emissions of N2O up to 7 months post-Georges ranged from 5.92 to 4.26 ng cm2 h1 and
averaged five times greater than fluxes previously measured at the site. N2O emissions 27
months after the hurricane remained over two times greater than previously measured
fluxes. Soil ammonium pools decreased after Georges and remained low. The first year
after the hurricane, nitrate pools increased, but not significantly when compared against
a single measurement made before the hurricane. Soil moisture and temperature did not
differ significantly in the two sampling periods. These results suggest that hurricanes
increase N2O fluxes in these forests by altering soil N transformations and the relative
availabilities of inorganic nitrogen.

Humid tropical forests are often characterized by
large nitrogen (N) pools, and are known to have
large potential N losses. Although rarely measured,
tropical forests likely maintain considerable biological
N fixation (BNF) to balance N losses. We
estimated inputs of N via BNF by free-living microbes
for two tropical forests in Puerto Rico, and
assessed the response to increased N availability
using an on-going N fertilization experiment.
Nitrogenase activity was measured across forest
strata, including the soil, forest floor, mosses, canopy
epiphylls, and lichens using acetylene (C2H2)
reduction assays. BNF varied significantly among
ecosystem compartments in both forests. Mosses
had the highest rates of nitrogenase activity per
gram of sample, with 11 ± 6 nmol C2H2 reduced/g
dry weight/h (mean ± SE) in a lower elevation
forest, and 6 ± 1 nmol C2H2/g/h in an upper elevation
forest. We calculated potential N fluxes via
BNF to each forest compartment using surveys of
standing stocks. Soils and mosses provided the
largest potential inputs of N via BNF to these ecosystems.
Summing all components, total background
BNF inputs were 120 ± 29 lg N/m2/h in
the lower elevation forest, and 95 ± 15 lg N/m2/h
in the upper elevation forest, with added N significantly
suppressing BNF in soils and forest floor.
Moisture content was significantly positively correlated
with BNF rates for soils and the forest floor.
We conclude that BNF is an active biological process
across forest strata for these tropical forests,
and is likely to be sensitive to increases in N
deposition in tropical regions.

Topography may affect soil microbial processes, however, the use of topographic data to model and predict the spatial
distribution of soil microbial properties has not been widely reported. We studied the effect of topography on the activity of
denitrifiers under different hydrologic conditions in a typical agroecosystem of the northern grasslands of North America using
digital terrain modelling (DTM). Three data sets were used: (1) digital models of nine topographic attributes, such as elevation,
slope gradient and aspect, horizontal, vertical, and mean land surface curvatures, specific catchment area, topographic, and
stream power indices; (2) two soil environmental attributes (soil gravimetric moisture and soil bulk density); and (3) six
attributes of soil microbial activity (most probable number of denitrifiers, microbial biomass carbon content, denitrifier enzyme
activity, nitrous oxide flux, denitrification rate, and microbial respiration rate). Linear multiple correlation, rank correlation,
circular–linear correlation, circular rank correlation, and multiple regression were used as statistical analyses. In wetter soil
conditions, topographically controlled and gravity-driven supply of nutritive materials to microbiota increased the
denitrification rate. Spatial differentiation of the denitrification rate and amount of denitrifying enzyme in the soil was
mostly effected by redistribution and accumulation of soil moisture and soil organic matter down the slope according to the
relative position of a point in the landscape. The N2O emission was effected by differentiation and gain of soil moisture and
organic matter due to the local geometry of a slope. The microbial biomass, number of denitrifiers, and microbial respiration
depended on both the local geometry of a slope and relative position of a point in the landscape. In drier soil conditions,
although denitrification persisted, it was reduced and did not depend on the spatial distribution of soil moisture and thus land
surface morphology. This may result from a reduction in soil moisture content below a critical level sufficient for transient
induction of denitrification but not sufficient to preserve spatial patterns of the denitrification according to relief. Digital terrain
models can be used to predict the spatial distribution of the microbial biomass and amount of denitrifying enzyme in the soil.
The study demonstrated a feasibility of applying digital terrain modelling to investigate relations of other groups of soil
microbiota with topography and the system ‘topography–soil microbiota’ as a whole.

Plants and animals exploit the soil for food and shelter and, in the
process, affect it in many different ways. For example, uprooted trees may break up
bedrock, transport soil downslope, increase the heterogeneity of soil respiration rates,
and inhibit soil horizonation. In this contribution, we review previously published
papers that provide insights into the process of bioturbation. We focus particularly on
studies that allowus to place bioturbation within a quantitative framework that links the
form of hillslopes with the processes of sediment transport and soil production. Using
geometrical relationships and data from others’ work, we derive simple sediment flux
equations for tree throw and root growth and decay.

Labile carbon is the fraction of soil organic carbon with most rapid turnover times and its oxidation drives the flux of CO2 between soils and atmosphere. Available chemical and physical fractionation methods for estimating soil labile organic carbon are indirect and lack a clear biological definition. We have modified the well-established Jenkinson and Powlsons fumigationincubation technique to estimate soil labile organic carbon using a sequential fumigationincubation procedure. We define soil labile organic carbon as the fraction of soil organic carbon degradable during microbial growth, assuming that labile organic carbon oxidizes according to a simple negative exponential model. We used five mineral soils and a forest Oa horizon to represent a wide range of organic carbon levels. Soil labile organic carbon varied from 0.8 mg/g in an Entisol to 17.3 mg/g in the Oa materials. Potential turnover time ranged from 24 days in an Alfisol to 102 days in an Ultisol. Soil labile organic carbon contributed from 4.8% in the Alfisol to 11.1% in the Ultisol to the total organic carbon. This new procedure is a relatively easy and simple method for obtaining indices for both the pool sizes and potential turnover rates of soil labile organic carbon and provides a new approach to studying soil organic carbon.

Direct and Optical Assay of Leaf Mass of the Lower Montane Rain Forest of Puerto Rico
Howard T. Odum, B. J. Copeland and Robert Z. Brown
Proceedings of the National Academy of Sciences of the United States of America
Vol. 49, No. 4 (Apr. 15, 1963), pp. 429-434

Dwarf mangroves on peat substrate growing
in eastern Puerto Rico (Los Machos, Ceiba State
Forest) were analyzed for element concentration, leaf
sap osmolality, and isotopic signatures of C and N in
leaves and substrate. Mangrove communities behind
the fringe presented poor structural development with
maximum height below 1.5 m, lacked a main stem,
and produced horizontal stems from which rhizophores
developed. This growth form departs from
other dwarf mangrove sites in Belize, Panama, and
Florida. The dwarf mangroves were not stressed by
salinity but by the low P availability reflected in low
P concentrations in adult and senescent leaves. Low P
availability was associated with reduced remobilization
of N and accumulation of K in senescent leaves,
contrasting with the behavior of this cation in
terrestrial plants. Remobilization of N and P before
leaf abscission on a weight basis indicated complete
resorption of these nutrients. On an area basis,
resorption was complete for P but not for N. Sulfur
accumulated markedly with leaf age, reaching values
up to 400%, compared with relatively modest accumulation
of Na (40%) in the same leaves. This
suggests a more effective rejection of Na than sulfate
at the root level. Dwarf mangrove leaves had more
positive d13C values, which were not related to
salinity, but possibly to drought during the dry season
due to reduced flooding, and/or reduced hydraulic
conductance under P limitation. Negative leaf d15N
values were associated with low leaf P concentrations.
Comparison with other R. mangle communities
showed that P concentration in adult leaves below
13 mmol kg-1 is associated with negative d15N
values, whereas leaves with P concentrations above
30 mmol kg-1 in non-polluted environments had
positive d15N values.

We used a humid tropical elevation gradient to examine the relationships among climate, edaphic conditions, belowground carbon storage, and soil respiration rates. We also compared open and closed canopy sites to increase the range of microclimate conditions sampled along the gradient, and determine the effects of canopy openings on C and P storage, and C dynamics. Total soil C, the light C fraction, and all of the component fractions of the P pool were significantly related to soil moisture, and all but total soil C were also significantly related to temperature. Both labile and recalcitrant soil P fractions were negatively correlated with the light C fraction, while the dilute HCl-extractable P pool, generally thought of as intermediate in availability, was positively correlated with light C, suggesting that P may play an important role in C cycling within these systems. Total fine root biomass was greatest at 1000 m elevation and lowest at 150 m, and was strongly and positively correlated with soil moisture content. Soil respiration rates were significantly and negatively correlated with fine root biomass and the light C fraction. In forested sites, soil respiration rates were strongly and negatively correlated with total belowground C pools (soils 1 roots 1 forest floor). Belowground C pools did not follow the expected increasing trend with decreases in temperature along the gradient. Our results indicated that in humid tropical forests, the relationships among soil C and nutrient pools, soil respiration rates, and climate are complex. We suggest that frequent and prolonged anaerobic events could be important features of these environments that may explain the observed trends.